The present invention relates to a zoom lens apparatus particularly for actively controlling the lens position for variation of the image magnification and focusing.
The present invention furthermore relates to compact zoom lenses with high speed and with a high variable magnification ratio which are particularly suited to video cameras.
Recently, a control method is also used in zoom lenses such as that in place of lens position control by a cam, in which the lens group position is operated according to the detected value of each lens position detecting means, free of restrictions such as defects caused by the shape of the cam groove.
For example, a zoom focusing method is indicated in Japanese Patent Application Publication No. 52-15226, wherein a first detection means for detecting the set focal length of the variable magnification lens system and a second detection means for detecting the focus condition of the lens are installed and the lens group position is determined and controlled by operations according to the detected values of the first and second detection means so as to keep the focus point within the depth of focus.
However, the above prior art causes problems such as those below when detected values are changed as time elapses.
(1) When the detected value is changed faster than the response speed of the lens group position setting means, it is difficult to catch up with the change of focus and to compensate it by controlling operation. PA1 (2) A position setting means which can follow expected high speed changes in the detected value is large in scale and expensive, and cannot be mounted to amateurs' cameras. PA1 .nu..sub.2- : average value of Abbe numbers of the negative lenses of the second lens component; PA1 .nu..sub.2+ : Abbe number of the positive lens of the second lens component; and PA1 n.sub.3 : refractive index of the positive lens of the third lens component. PA1 f.sub.w : Focal length of the entire lens system at the wide angle end PA1 f.sub.2 : Focal length of the second lens component PA1 F.sub.w : F-number at the wide angle end of the entire lens system PA1 Z: Zoom ratio PA1 f.sub.w : Focal length of the entire lens system at the wide angle end PA1 f.sub.3A : Focal length of the single lens of the third lens group which has an aspheric surface PA1 F.sub.w : F number at the wide angle end of the entire lens system PA1 .DELTA..sub.3 : Deformation value from the master sphere at the clear aperture radius position of the asphere of the third lens component, the radius of curvature of said master sphere is equal to the radius of axial curvature of said aspheric surface PA1 f.sub.w : Focal length of the entire lens system at the wide angle end PA1 f.sub.4 : Focal length of the fourth lens component PA1 f.sub.w : Focal length of the entire lens system at the wide angle end PA1 f.sub.5 : Focal length of the fifth lens component
Furthermore, zoom lenses with high speed and with a high variable magnification ratio suited to video cameras are indicated in Japanese Patent Application Laid-Open Nos. 62-24213 and 63-123009, wherein four lens components with refractive powers of positive, negative, positive, and positive are mounted sequentially from the object side, the first and third lens components are fixed during variation of the image magnification, the second component is moved in one direction so as to perform the variation of the image magnification, and the fourth lens component is moved back and forth so as to compensate variations in the focal point caused by the variation of the image magnification.
In zoom lenses by this method, the movement distance of the fourth lens component for correcting the image surface position due to the magnification ratio is comparatively large, and when the entire system is scaled down so as to make the lens system compact, variations in the aberration caused by the variation of the image magnification cannot be fully corrected.
Furthermore, in zoom lenses by this method, it is well known that the number of lenses is comparatively small though each lens has a high magnification ratio and a large aperture ratio, and the distance between the first lens and the diaphragm can be shortened because the third lens component does not move during zooming. Therefore, the diameter of the front lens can be made comparatively small. However, if the lens system is made more compact and the magnification ratio is increased, the performance is inevitably degraded. Particularly, the distortion and astigmatism aberration are increased.